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Snohomish County, WA Accessory Structures: ADU Rules (2026)

Some Restrictions

Compiled from the official code textEditor Martyn O'NeillLast verified September 2026

ADU rules in Snohomish County, WA, also called accessory dwelling unit regulations or granny flat ordinances, cover setbacks, owner-occupancy, parking, and permit requirements.

Key Facts

Governing section: SCC 30.28.010
Max ADU floor area: 1,200 sq ft
Urban zone ADU count: Up to 2 per single-family lot
Rural zone ADU count: 1 per lot
Urban lot coverage bump: Increased to 55%
R-5 detached ADU rule: Barred on lots under 5 acres

ADU Eligibility Quick-Check

An Accessory Dwelling Unit (ADU) is a smaller, independent living unit on the same lot as a single-family home: converted garages, detached cottages, basement or attic conversions, and attached additions all qualify. Most states have streamlined ADU permitting since 2020, but local rules still control size, height, and setback.

Is your lot zoned single-family or multi-family residential?
Does your lot have at least 1,200 sq ft of buildable space?
Is your existing home owner-occupied (in most states this is not required, but some require it for ADU permits)?
Will the ADU stay under your state's size cap (commonly 850-1,200 sq ft for detached)?
Can you provide one off-street parking space (waived in many states within 0.5 mi of transit)?

If you answered 'yes' to most of these, you likely qualify in Washington under recent ADU streamlining laws. The Snohomish County, WA rules below cover local size, height, and setback requirements.

Summary

SCC 30.28.010 caps ADU floor area at 1,200 square feet and lets urban single-family lots host up to two accessory units, in any mix of attached or detached, since the county's 2025 rule changes. Rural lots get only one, and detached units are barred on R-5 lots under five acres.

City-specific rules exist: Edmonds, Lynnwood, Everett, and Marysville have their own adu rules rules that differ from Snohomish County's county-level regulations. If you live in one of those cities, check the city-specific page instead.

Accessory dwelling units are allowed under SCC 30.22.100, 30.22.110, 30.22.120, and this section. (1) General standards. All accessory dwelling units shall comply with the following standards: ... (e) The floor area of an accessory dwelling unit shall not exceed 1,200 square feet. Floor areas shall be exclusive of garages, porches, unheated storage areas, and unfinished basements... (2) Urban zones. Accessory dwelling units are permitted uses in the urban zones pursuant to SCC 30.22.100 on lots with a single family, single family attached, or duplex dwelling. (a) When the principal unit is a single family dwelling, a maximum of two accessory dwelling units are permitted on the lot containing the principal unit...

Source: SCC 30.28.010 - Accessory Dwelling UnitsView official code

Official source re-checked September 8, 2026: no newer edition of the code had been published (publisher’s edition: current through legislation passed August 12, 2026).

Full Breakdown

120 to govern where and how accessory dwelling units may be built. 26 SCC's off-street parking standards. In the urban zones, subsection (2) allows up to two accessory dwelling units per lot when the principal unit is a single-family home, in any combination of one attached and one detached, two attached, or two detached units possibly sharing one structure; a single-family attached principal unit gets one ADU, and a duplex principal unit also gets up to two. Urban ADU development also unlocks modified bulk standards: maximum lot coverage rises to 55 percent, setbacks from private roads drop to five feet, and public-road setbacks for the ADU itself are reduced to as little as five feet on public roads 60 feet wide or wider under subsection (2)(d).

An existing nonconforming structure can be converted into an ADU even if it does not meet current setback or lot coverage rules, subject to conditions the director may add for health, safety, and welfare. In rural, resource, and other zones, subsection (3) allows only one ADU per lot with a legally established single-family dwelling, and it flatly prohibits detached ADUs on lots that do not meet the zone's minimum lot area, specifically barring detached units on R-5 lots under five acres and on RC lots under 100,000 square feet; a mobile home may serve as a detached ADU only on lots of 10 acres or more, and rural ADUs must share the principal dwelling's driveway.

Violations & Fines

Building or occupying an ADU beyond the unit count, size, or setback allowed under SCC 30.28.010 is prosecuted as a Title 30 zoning violation under chapter 30.85 SCC. A notice of violation carries a $500 penalty for a first noncommercial violation, escalating under the SCC 30.85.170 schedule toward a $10,000 cap, and the department can also require removal or bring the structure into a legal configuration before it can be occupied.

Frequently Asked Questions

How many accessory dwelling units can I build on an urban lot in Snohomish County?

SCC 30.28.010(2) allows up to two ADUs on a lot with a single-family or duplex principal dwelling, in configurations such as one attached and one detached, two attached, or two detached units, and one ADU when the principal unit is a single-family attached dwelling.

What is the maximum size for an ADU in unincorporated Snohomish County?

SCC 30.28.010(1)(e) caps ADU floor area at 1,200 square feet, not counting garages, porches, unheated storage areas, or unfinished basements, and this limit applies in every zone where ADUs are allowed.

Can I build a detached ADU on a rural R-5 lot?

Only if the lot is at least five acres. SCC 30.28.010(3)(a)(i)(A) specifically prohibits detached accessory dwelling units on R-5 zoned lots smaller than five acres, and on RC zoned lots under 100,000 square feet.

Sources & Official References

[1]SCC 30.28.010 Accessory dwelling units

Other rules in Snohomish County

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All Snohomish County rules

How Snohomish County compares: Easiest Cities to Build an ADU·Compare Snohomish County to another location·View the Washington accessory structures overview

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ADU Rules in Cities Across Snohomish County

Edmonds, WA Everett, WA Lynnwood, WA Marysville, WA